THE TRUE PRINCIPLES OF CLASSIFICATION. 51 



arran2:ement accordino; to their natural affinities that can be ascertained 

 by that study. Future studies, discoveries of new rocks, and other causes 

 are liable, in this science as in botany and zoology, to change the particular 

 arrangement ; but the principles and methods, so far as they are natural, will 

 remain the same. 



The natural method is sufficiently elastic to allow of the incorporation 

 of whatever new divisions future investigations may require ; it can never 

 expect to be fixed and rigid, until the sum of human knowledge shall be 

 complete. These changes will result simply in removing the artificial por- 

 tions which imperfect knowledge has incorporated with it, and bring the 

 classification nearer and nearer to the perfect natural system. If none of 

 tlie system here adopted is natural, then all will in time be removed. 



The important principle that underlies the natural classification, as here 

 advocated, is the belief that the older rocks now classed as distinct species are 

 rocJcs that once u'cre identical with their yonnr/er prototypes — the presort differences 

 being due to alteration, and conditions of crijstallization. 



Standing next to this, is the belief in the chimgeaUvness of the mineral con- 

 stitution of the rocJcs, and the feeling that no classification should be placed 

 entirely on so uncertain a foundation. 



As a deduction from the preceding discussion, the following statements 

 may be taken as guides in describing and classifying our rocks : — 



Sectiox VIII. — The Principles of Classification. 



1. Ix the study of rocks, we should begin with the younger and glassy 

 state, and follow the gradations step by step to the most crystalline one in 

 the series — from the least altered to the most altered forms — tracing every 

 change, and studying their history in their tufaceous, poroditic, raetamorphic, 

 or any other state in which they or their remains can exist. 



2. Any rocks that can be followed in this way between certain limits, 

 whatever may be the changes they have undergone, form a species. In 

 every shape they should be retained under the specific name ; the various 

 modifications, when of sufficient importance, being regarded as varieties, 

 and named as such. 



3. All the petrological, lithological, and chemical characters should be 

 used in determining rock species ; that is, the rock as a whole and in all 

 its I'elations should be considered. 



